As a rule, hollow plastic articles are produced in blow molding processes in such a way that the starting material which is plasticized in an extruder by the action of pressure and heat is molded in a so-called "blow head" or "tube head" downstream of the extruder into a tubular skein and squeezed vertically downwards from the nozzle of the blow head, from which sections are introduced individually as parisons into the mold cavity of a divided and opened blow mold, the parisons being expanded, after the blow mold has been closed, into hollow plastic articles of the desired shape by introducing a blow medium by means of a blow pin or a blowing needle until contact is established with the wall of the mold cavity. The parts of the blow molds, most of the time blow mold halves, are replaceably mounted on mold supporting plates or closing plates which, guided on struts, are displaceable, most of the time hydraulically, relative to one another by opening and closing the blow mold. The unit which consists of the mold supporting plates, the guide struts, the mold parts and the movement mechanism is designated as a " mold closing support". In most of said machines, the extruder with the blow head is at a standstill while the whole mold closing support can be moved back and forth between the blow head and the blow unit consisting of the blow pin or blowing needle and the movement mechanism therefor, i.e., the empty opened mold can be moved towards the blow head for receiving a parison and then moved back into the so-called "blowing station" in its closed state with a parison being supported in the cavity of the blow mold.
Although such a blow molding machine normally permits not only the production of rotationally symmetrical articles, it permits only the production of such articles that have a main axis only, e.g. bottles, small bottles, canisters, wide-necked vessels (resembling jam jars), and so-called "technical parts" having one main axis, such as tanks, receptacles for the front and rear window cleaning liquid, braking oil vessels, armrests etc., in the car manufacturing field. Deviating forms, such as bottles with a handle, watering cans, etc., can then be produced in the already described process when the areas which are positioned between the main body and the molded part, can be "pinched off", i.e., for instance, in the case of a watering can the area inside the handle and in the case of the spout the area between the main body and this spout. This means the possibility of squeezing these areas of the tubular parison in the blow mold during closing of the mold in such a way that they can easily be removed from the mold after the sufficiently cooled hollow plastic article has been removed.
When, apart from the production of hollow plastic articles which are predominantly used for packing or for accommodating liquids and bulk materials, i.e., all kinds of bottles, tablet tubes, cans, canisters, etc., the plastics blowing technology started to deal with the manufacture of technical parts, the desire for being capable of producing hollow plastic articles having a plurality of axes arranged at angles and spatially with one another continually increased. Tubular pieces that are bent once or several times could here be mentioned as the most simple example thereof, which pieces, however, were very difficult to produce or could not be produced at all with the prior-art machines, depending on the respective form and size.
The difficulty encountered most of the time consists, not in the expansion of the parison inside the mold, but in the introduction thereof into the blow mold. It is here no longer possible to squirt only one tubular piece with a sufficient length vertically downwards with the aid of the blow head, to move said tubular piece still attached to the supply between the opened blow mold with a vertical parting plane, then to close the mold and to separate the parison inside the closed mold from the supply by closing the mold or thereafter. When hollow plastic articles are produced with a plurality of axes, the parison must rather be inserted into the correspondingly multiaxial and partial mold cavity of a blow mold part and said blow mold part must be positioned horizontally or obliquely to this end. Since the mold cavity corresponding to the hollow plastic article to be made has more than one axis, and since these are at angles within the space, i.e. the cavity may consist of a plurality of sections extending upwards and downwards at both sides, either the horizontally or obliquely positioned mold cavity must be moved back and forth and also upwards and downwards during the straight-lined discharge of a parison or these movements have to be performed by the extruder when the mold is at a standstill. Since in the prior art the whole mold closing support or the extruder with the blow head is moved in three axes perpendicular to one another, considerable weights have to be moved even in the case of relatively small articles to be produced, because both the mold closing supports and the extruders weigh tons--even in the case of small machines. An example of this prior art is given in U.S. Pat. No. 4,239,474. In the blow molding machine of this publication the mold is arranged with a horizontal parting plane and can only be moved in the Z axis within its mold closing support apart from being opened and closed as a whole, i.e., it can only be lifted or lowered. The movements in the X and Y axes are performed by displacements of the extruder with the coupled blow head. In a modified embodiment, the movement in the Z axis is not performed by lifting or lowering the mold closing support, but by vertically displacing the blow head relative to the extruder. While with the first-mentioned type the horizontally split mold can be opened and the horizontally positioned lower mold half becomes accessible to the extruder after the upper half has been unfolded, the horizontally positioned lower mold half is moved in the second embodiment along a straight or curved path out of the area of the upper mold half vertically positioned thereabove so as to become accessible to the moved extruder with the blow head. This apparatus has the fundamental disadvantage that a plastically moldable tubular parison of plastics which is suited for the subsequent blowing into a hollow plastic article and which is discharged horizontally from the blow head nozzle is always more difficult to handle than a tube vertically squirted downwards from the nozzle of the blow head, and has to be introduced into a mold cavity, a difficulty which becomes even greater with mold cavities for multiaxial hollow articles. In an embodiment in which the horizontally positioned lower blow mold half is moved along a curved path, normally a circular path, and filled into a station and closed in another station, it is disadvantageous that the time is relatively long until the upper mold half is moved onto the lower mold half, whereby the mold is closed. The parison remains, so to speak, in the lower mold half and is cooled on the contact surface with the mold cavity with time, whereby problems might arise during the subsequent blowing process. It should here be noted that the mold halves in all blow molding machines are always cooled by inner cooling channels for rapidly cooling the finish-blown part and therefore remain relatively cool all the time, i.e. also when moving from one station to the next one.
This has been regarded as such a considerable disadvantage that in some cases preference was given to the production of multiaxial hollow articles not with the aid of such a troublesome apparatus, but with pinched-off areas according to the method described above with reference to the example of the production of watering cans if the axial extension of said articles was not too complicated. However, it goes without saying that excessively large pinched-off surfaces are obtained at any rate with an amount of material that is very great in comparison with the amount of material of the finished hollow article and has to be severed after removal from the mold and cooling, which is of course a great disadvantage.